Conventionally, there has been an apparatus for detecting a decrease in tire air pressure (DWS) for detecting a decreased pressure of the tire based on the rotation speed information (wheel speed) regarding four tires attached to a vehicle. This apparatus uses a principle according to which a tire having a decreased pressure shows a reduced outer diameter (tire dynamic loaded radius) compared to that of tires having a normal internal pressure and thus shows an increased rotation angular velocity compared to that of other tires having a normal internal pressure. Various methods have been suggested to determine a decreased pressure. In the case of a method of detecting a decreased internal pressure based on a relative difference in the rotation angular velocities of the tires for example, the following formula (1) is used for a determination value (see Patent Literature 1 for example).DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)  (1)In the formula, F1 to F4 mean the rotation angular velocities of a left front tire (FL), a right front tire (FR), a left rear tire (RL), and a right rear tire (RR), respectively.
In the case of the method disclosed in Patent Literature 1, this determination value DEL is calculated, and then subjected to a required correction such as a cornering correction and to a rejection of unnecessary data. When the resultant DEL exceeds a threshold value set in advance, it is determined that there is occurred a tire deflation.
However, under running conditions influenced by a high torque by a driving wheel such as an upward slope or a towing running, the driving wheel tire tends to slip. Thus, there may be a case where, even when the air pressure is normal, this air pressure is erroneously determined as a decreased pressure and a false alarm is issued.
In order to prevent such a false alarm, a method has been suggested according to which the torque information from an engine control unit is used to calculate the torque of the driving wheel. When a difference between this driving wheel torque and a driving wheel torque calculated in advance that is required for the vehicle to run on a flat road at a fixed speed (hereinafter also referred to as “standard driving wheel torque”) exceeds a threshold value set in advance, the data obtained at this time is assumed as unnecessary data and is rejected (see Patent Literature 2).
In the case of the method disclosed in Patent Publication 2, when the driving wheel torque is within the hatched area in FIG. 6 (i.e., when the driving wheel torque is equal to or higher than the positive threshold value Tth1 set in advance and when the driving wheel torque is equal to or lower than the negative threshold value Tth2 set in advance), the determination of a decrease in tire air pressure is prohibited. When the driving wheel torque is within areas other than the hatched area, the determination of a decrease in tire air pressure is allowed. The rejection processing as described above reduces the variation in the decreased pressure determination value when the vehicle is running on an upward slope or is performing a towing running, thus preventing a false alarm from occurring.
In the case of the method disclosed in Patent Literature 2, however, when the vehicle is running on such a road that includes both of an upward slope (or a downward slope) and a curve, the variation in the decreased pressure determination value is caused even when the air pressure is normal, which may cause a false alarm.
In view of the above, the present applicant has suggested to change, as the driving wheel torque is more away from the standard driving torque (for which a difference between the driving wheel torque and the standard driving torque is called a Δtorque. See FIG. 7.), the threshold value for rejecting a lateral acceleration (lateral G) in order to prevent the data when the vehicle is turning while moving upward or downward from being used to determine a decrease in tire air pressure (German Patent Application No. 102009020784.8. Hereinafter referred to as the invention of the prior application).
In the method according to the invention of the prior application, a yaw rate value is received from the sensor attached to the vehicle and the lateral G can be calculated based on the yaw rate value and the vehicle running speed. In FIG. 8(a) or FIG. 8(b), when the torque and the lateral G are within the hatched area, the determination of a decrease in tire air pressure is allowed. When the Δtorque and the lateral G are within areas other than the hatched area on the other hand, the determination of a decrease in tire air pressure is prohibited.
The difference between FIG. 8(a) and FIG. 8(b) will be described. In the case of FIG. 8(a), the possibility can be reduced where an alarm is issued even when there is no decrease in tire air pressure. However, the range of usable data is narrow and thus more data is rejected, thereby suppressing data used for the determination of a decreased pressure from being collected. This consequently may cause a case where a long time is required to issue an alarm for notifying a decrease in tire air pressure. In the case of FIG. 8(b) on the other hand, the range of usable data is wide so that data can be used when the lateral G is low even when Δtorque is high. Thus, FIG. 8(b) requires a shorter time to issue an alarm for notifying a decrease in tire air pressure than in the case of FIG. 8(a).
In the case of the method according to the invention of the prior application, data when the vehicle is turning while moving upward or downward can be rejected and thus such data can be obtained that is under more ideal conditions (i.e., under conditions close to those where the vehicle is running straight on a flat road). This can consequently reduce the variation in the decreased pressure determination value DEL to thereby prevent a false alarm from occurring.